Similar low speed driveline shudder in my 1978 280CE 60K miles turned out to be the centre bearing mount.......but I described it as a 'flapping' which stopped at ~ 20mph

Put the back wheels up on stands and push/pull the drive shaft from the centre bearing to the diff.......I got obvious excessive movement.........I ran it carefully in drive on the rear stands and saw the excessive shaft movement and knocking of the bearing in its mount

Problems with the flex discs should show up with this test (I was expecting the flex discs to be the problem)

Centre bearing & mount replacement is not difficult or expensive..........if your problem is the universal it is a different story as the whole shaft needs replacing

I can help with more details on centre bearing replacement if required